Performance of Unified Power Quality Conditioner

ABSTRACT

Power electronics is playing an important role in transmission and utilization of electrical power due to its capability of processing electric power in most efficient and cost-effective way. However, the nonlinear characteristics of power electronic devices give rise to two important limitations; they generate harmonics and draw lagging current from the utility. In recent years unified power quality conditioner (UPQC) is being used as a universal active power conditioning device to compensate both harmonics as well as reactive power. UPQC is an advanced version of unified power flow controller (UPFC). The performance of UPQC mainly depends upon how quickly and accurately compensation signals are derived. The UPQC mitigates harmonics and provides reactive power to the power systems network so as to improve the power factor close to unity.

The UPQC is a combination of shunt active and series active power filters connected through a dc bus. The shunt active filter of UPQC acts as a current source for injecting compensating current through a shunt transformer, whereas, the series active filter acts as a voltage source for feeding compensating voltage through a series transformer. The aim of the dissertation work is to study the control strategies of UPQC based on PI controller and fuzzy logic controller in detail.

In the case of PI controller, the dc link voltage is sensed at regular intervals and is compared with a reference value. The error signal thus derived is processed in a PI controller. A limit is put on the output of the controller to ensure that the shunt active power filter supplies active power of the load through the series active power filter.

The fuzzy logic controller is basically nonlinear and adaptive in nature. This gives a robust performance in the cases where the effects of parameter variation of controller are also taken into consideration. It is a well established fact that the fuzzy logic controller yields results that are superior to those obtained as compare to those obtained through conventional controllers such as PI and PID because of the fact that it is based on linguistic variable set theory and does not require a mathematical model. Generally, the input variables are error and rate of change of error. If the error if coarse, the fuzzy controller provide coarse tuning to the output variable and if the error is fine it provides fine tuning of the output variable.

The present thesis investigates PI controller and fuzzy logic controller as concerned to UPQC application for power quality improvement. The UPQC is studied and its advantages over conventional APFs and UPFC are discussed in detail. The relevant mathematical models and equations to explain the working of UPFC are derived for both the cases (PI controller and fuzzy logic controller).The relevant simulations are carried out using MATLAB/Simulink.

The result obtained reveals that the fuzzy logic controller gives better dynamic performance than the PI controller for power quality improvement.

Chapter 1: INTRODUCTION

1.1 Theory

The electrical power system consisting of generation, transmission and distribution system are based on alternative voltage and currents. When linear load consisting of inductances, capacitances and resistances are connected to the power system the sine wave is preserved and the system components are said to be linear. Traditionally, linear loads consume major part of electrical power. However situation has changed now as more and more electrical power are being developed using power electronic devices due to their energy efficiency and control. Power electronic devices possess inherent non linear characteristics. The nonlinear characteristics of this devices results in two important limitations, drawing of large reactive volt-amperes and injection of harmonics into the utility. Large reactive volt-amperes drawn from the utility leads to increase voltage drops at various buses. The harmonics increase the losses in transformers, generators, motors, capacitors, conductors, etc. some of the control devices interfaced with the utility starts malfunctioning due to excessive harmonic currents.

As the non linear load consists of the major portion of the total load for the last two three decades, reactive power compensation and harmonic filtering have received a great deal of attention. To restrict the consumers against excessive loading VARs and harmonics, stricter standards has been laid down by the utilities. Most popular among them is standard 519-1992 [1].

Static VAR compensators using thyristor switched capacitors (TSC) and thyristor control inductors (TCI) [2], [3] have been traditionally used for reactive power compensation. As the VAR generated in these schemes are directly proportional to the energy storage capability of capacitors and inductors, there is considerable increase in the size of these elements when the VARs to be compensated are large. Moreover TSC and TCI produce additional current harmonics. Therefore shunt passive filters require filtering them out. Active power filter (APF) using voltage or current source inverter can be used for reactive power compensation and harmonic filtering together. The major advantage of using voltage source or current source inverter is that the size of the energy storing element is drastically reduced as compare to TSC or TCI.

The shunt APF is the most commonly used APF. The power circuit of shunt APF is shown in Fig. 1.1. In shunt APF, a reactive volt ampere calculation estimates the real component of the load current, Ipland then determines the resistive component of the load current by subtracting Ipl from IL(Iql= IL-Ipl). If nonlinearity present in the load current, it is present in Iql as well. Since compensation current Icomp is made to follow Iql, load harmonics also get eliminated. Apart from shunt APF various other APF topologies such as series active filter, hybrid series active filter and power line conditioner have been proposed in the literature.

The series active filter as shown in Fig. 1.2 is connected in series with supply mains using a matching transformer. Its limitation is that the presence of active impedance in series with source produces voltage harmonics.

IL = Ipl +Iql

Source

Icomp = Iql

Source

Source side Series transformer Load side

Shunt transformer

DC

Link Capacitor

Converter 1 converter 2

Using combine series APF and shunt APF unified power flow controller (UPFC) realized, which performs active power compensation, reactive power compensation and phase angle regulation. UPFC believed to be the most complete power conditioning device. But as the time changes, problem also changes. Now days electrical engineers facing problem regarding harmonic compensation, voltage sag and voltage flickering and UPFC is not able to overcome these problems. So a new concept based on UPFC derived called unified power quality conditioner (UPQC) as shown in Fig. 1.3, which performs all the basic functions of UPFC in addition it also compensate for current /voltage harmonics with constant voltage maintenance at load terminals.

1.2 Unified Power Quality Conditioner

The UPQC is the most versatile and complex of the FACTS devices, combining the features of the STATCOM and the SSSC. The UPQC can provide simultaneous control of all basic power system parameters, transmission voltage harmonic compensation, impedance and phase angle. It is recognized as the most sophisticated power flow controller currently, and probably the most expensive one. The basic components of the UPQC are two voltage source inverters (VSIs) sharing a common dc storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer. A basic UPQC functional scheme is shown in Fig.1.3. The series inverter is controlled to inject a symmetrical three phase voltage system of controllable magnitude and phase angle in series with the line to control active and reactive power flows on the transmission line. So, this inverter will exchange active and reactive power with the line. The reactive power is electronically provided by the series inverter, and the active power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to demand this dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPQC is equal only to the losses of the inverters and their transformers. The remaining capacity of the shunt inverter can be used to exchange reactive power with the line so to provide a voltage regulation at the connection point [8]-[11].

A conventional UPQC topology is comprised of the integration of two active power filters connected back to back to a common dc link bus. A simple block diagram of a typical UPQC is shown in Fig. 1.4. The first active filter connected in series through an injection transformer is commonly termed as series filters (SF). It acts as a controlled voltage generator. It has capability of voltage imbalance compensation, voltage regulation and harmonic compensation at the utility-consumer PCC. In addition to this, it provides harmonic isolation between a sub-transmission system and a distribution system.

A UPQC consists of combination of shunt active filter and series active filter with a common dc link as shown in Fig. 1.4. The dc link capacitor allows the active power generated by the shunt active filter and active power drawn by the series filter to be same. Further dc link capacitor increases or decreases with respect to rated voltage which depends upon power generated and absorbed by both active filter can be choosen independently which gives flexibility to the power outlet.

The performance of these active filters is based on three basic design criteria. They are:

1. Design of power inverter (semiconductor switches, inductances, capacitors, dc voltage);
2. PWM control method (hysteresis, triangular carrier, periodical sampling);
3. Method used to obtain the current reference or the control strategy used to generate the reference template.

Both series voltage control and shunt current control involve use of voltage source converters. Both these inverters each consisting of six IGBTs with anti parallel diode connected with each IGBT are operated in current control mode employing PWM control technique. Capacitor is used as an interface between the two back to back connected inverters and the voltage across it acts as the dc voltage source driving the inverters

The two VSI’s can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to regulate the voltage magnitude at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power flows on the transmission line. The UPQC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current into the transmission line. The shunt inverter can be controlled in two different modes:

(1) VAR Control Mode:The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the VAR reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required.

(2) Automatic Voltage Control Mode:The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value.. The series inverter controls the magnitude and angle of the voltage injected in series with the line to influence the power flow on the line. The actual value of the injected voltage can be obtained in several ways:

Direct Voltage Injection Mode:The reference inputs are directly the magnitude and phase angle of the series voltage.

Phase Angle Shifter Emulation mode: The reference input is phase displacement between the sending end voltage and the receiving end voltage.

Line Impedance Emulation mode: The reference input is an impedance value to insert in series with the line impedance.

Automatic Power Flow Control Mode:The reference inputs are values of active and reactive power to maintain the transmission line despite system changes.

A UPQC control strategy should preferably have following attributes:

(1) Shunt converter

1. Reactive power control by shunt current injection
2. Real power regulation through dc link capacitor
3. DC capacitor voltage regulation
4. Harmonic compensation

(2) Series converter

1. Real & reactive power control by series voltage injection
2. Voltage control
3. Phase angle regulation
4. Power factor correction

1.3 Characteristics of UPQC

Basic characteristics of UPQC are same as UPFC but UPQC in addition, performs active filtering. The operation of UPQC from the standpoint of conventional power transmission based on reactive shunt compensation, series compensation and phase angle regulation, the UPQC fulfill these functions there by meet multiple control objectives by adding injected voltage with appropriate magnitude and phase angle to the terminal voltage. Using phasor representation, basic UPQC control functions explained:

(1) Terminal Voltage Regulation

The change in voltage shown in Fig.1.5 is injected in phase or anti phase. UPQC with its series voltage control detects and calculates the required terminal voltage vo to be injected in series with the line to compensate both the dip and swell in the supply voltage.

vo + vo

vo

(2) Series Capacitive Compensation

Here, vpq = vc where vcis injected capacitive voltage in quadrature to the line current functionally it is similar to series capacitive and inductive line compensation attained by SSSC as shown in Fig. 1.6. Series inverter in combination with the insertion transformer produces the series injected voltage as calculated to mitigate the effects of the fluctuations of supply voltage by drawing the required power from the dc link.

vc

vo vo + vc

Fig. 1.6 Series capacitive compensation

(3) Transmission Angle Regulation

Here, vpq = v (δ) is injected with an angular relationship with respect to the voltage that achieves desire phase shift without any change in the magnitude as shown in Fig. 1.7. At any given transmission angle δ, the transmitted real power demand P and reactive power demand at transmission line sending end Qs and receiving end Qr can be freely controlled by UPQC

Vc vd

δ

vo

vo + vδ

(4) Multifunction Power Flow Control

This property is executed by simultaneous terminal voltage regulation, series capacitive line compensation and phase shifting as shown in Fig.1.8. This function makes UPQC unique device that performs all power quality improvement functions.

vc

Δvvδ

vpq

vo + δv + vc + vδ

(5) Active Filtering

The compensating shunt currents generated contain harmonic content of the load current but with opposite polarity such that when they are injected at the point of common coupling the harmonic content of supply current is effectively reduced. As discussed earlier in this chapter.

1.4 Aim of Work

This work deals with UPQC, which aims at the integration of series-active and shunt-active power filters. Fig. 1.3 shows the basic system configuration of such a UPQC. In this system, the power supply is assumed to be a three-phase, three-wire system. The two active power filters are composed of two 3-leg voltage source (VSI). The main purpose of the series-APF is harmonic isolation between a sub transmission system and a distribution system. In addition, the series-APF has the capability of voltage imbalance compensation as well as voltage regulation and harmonic compensation at the utility-consumer point of common coupling (PCC). Atthe same time, the main purpose of the shunt- APF is to absorb current harmonics, compensate for active power and reactive power injected by the load. Also, the voltage of the DC link capacitor is controlled to a desired value by the shunt-APF.

The aim of the dissertation is to design different control strategies for (UPQC), which is one of the major custom power solutions capable of mitigating the effect of supply voltage sag, swell, flicker and spikes at the load end or at the Point of Common Coupling (PCC). It also prevents load current harmonics from entering the utility and corrects the input power factor of the load. Further, the main aim of the dissertation is to implement a control strategy for UPQC, modeling of UPQC using simulink and to analyze the control strategy to use the series voltage injection and shunt current injection for UPQC control The control strategies used here are based on PI controller, fuzzy controller. The relative performance of the two controls is also studied.

The present work discusses the compensation principle and different control strategies (PI, Fuzzy) of the UPQC in detail [12]-[15]. The control strategies are modeled using MATLAB/Simulink. The performance of UPQC is examined by considering, a diode rectifier feeding an RL load (non linear load) that acts as a source of harmonics, to the system of concern. The performance is also observed by switching the extra RL load. The simulation results are listed in comparison of different control strategies and for the verification of result [16]-[18].

1.5 Organization of the Report

The report of the work done is organized as follows:

Chapter 2 gives brief overview of control strategy of UPQC. In this chapter introduction to dq theory, compensation strategy, basic control function and modeling of UPQC using PI controller discussed with results.

Chapter 3 discusses about fuzzy logic controller and implementation in UPQC. Membership functions, rule base table and surface viewer also discussed in this chapter.

Chapter 4 gives comparison studied between fuzzy logic controller and PI controller. Simulation results of both are discussed in detail with the help of table and graphs.

The last chapter 5 presents important conclusions and future work. Adequate references provided at the end of the chapter.

Chapter 2: CONTROL STRATAGEY FOR UNIFIED POWER QUALITY CONDITIONER

2.1 Introduction

Control strategy plays vital role in overall performance of power conditioner. Control strategy includes features like rapid detection of harmonic signals by maintaining higher accuracy, fast processing, and faster dynamic response of the controller. The control strategy can be realized using discrete analog and digital devices or advanced programmable devices, such as single chip micro computers, DSPs etc[10].

The control strategy determined by the appropriate switching pattern or signal obtained by compensating gate signal compared obtained by comparing with its reference

value. Since derivation of reference signal plays an important role in control strategy, many theories and techniques were proposed in recent years. There are number of control strategies were proposed among them dq method is used in the present work and discussed below:

2.2 dq Transformation

It is established that the active filter flows from leading voltage to lagging voltage and reactive power flows from higher voltage to lower voltage. Therefore both active and reactive power can be controlled by controlling the phase and the magnitude of the fundamental component of the converter voltage with respect to line voltage. dq theory provides an independent control of active reactive power by controlling phase and the magnitude of the fundamental component with respect to converter voltage

According to the dq control theory three-phase line voltages and line currents are converted in to its equivalent two-phase system called stationary reference frame. These quantities further transformed into reference frame called synchronous reference frame. In synchronous reference frame, the components of current corresponding to active and reactive power are controlled in an independent manner. This three-phase dq transformation and dq to three-phase transformation are discussed in detail in this chapter. The outer loop controls the dc bus voltage and the inner loop controls the line currents. The instantaneous real power at any point on line can be defined by:

p =vRIR + vBIb + vCIc (2.1)

And we can define instantaneous reactive voltage conceptually as a part of three phase voltage set that could be eliminated at any instant without altering p.

Reference frame theory based d-q model of shunt active filter is presented in this section. While dealing with instantaneous voltages and currents in three phase circuits mathematically, it is adequate to express their quantities as the instantaneous space vectors [10]. Vector representation of instantaneous three phase quantities R, Y and B which are displaced by an angle 2π/3 from each other is shown in Fig.2.1 [17].

β

B

90o R α

120o

Y

The instantaneous current and voltage space vectors are expressed in terms of instantaneous voltages and currents as:

v= [vRvYvB] I = [IR IY IB] (2.2)

Instantaneous voltages and currents on the RYB co ordinates can be transformed into the quadrature α, β coordinates by Clarke Transformation as follows:

vαvβv0.=TvRvYvB. (2.3)

IαIβI0.=TIRIYIB. (2.4)

Where Transformation matrix

T=2/31-1/2-1/203/2-3/21/21/21/2 (2.5)

Since in a balanced three-phase three-wire system neutral current is zero, the zero sequence current does not exist and zero sequence current can also be eliminated using star delta transformer.

These voltages in α-β reference frame can further be transformed into rotating d- q reference frame as Fig. 2.2.

β d

Y

R α

ω

B q

T1=cosωr-sinωrsinωrcosωr (2.7)

Where ωr is the angular velocity of the d- q reference frame as shown in Fig. 2.2.

The current components in the d- q reference frame can be similarly obtained using the α-β to d-q transformation matrix T1. The unit vector required for this transformation is generated using the grid voltage

2.3 Compensation Strategy

vc

iL

ic

VL

vs

As shown in Fig. 2.3,vs is the supply voltage. vc, Ic are the series compensation voltage, shunt compensation current and vL, iL are the load voltage and current respectively. The source voltage may contain negative, zero as well as harmonic components. The per phase voltage of the system can be expressed as:

va=v1pm+sinωtsinθ+valn+k=2∞Vaksin kωt + θka (2.8)

Where v1pa is the fundamental frequency positive sequence components, v1naand v10a are negative and zero sequence components respectively. The last term of equation represents the harmonic content in the voltage. In order for the load voltage to be perfectly sinusoidal and balanced, the series filter should produce a voltage of:

vah=v1an+v10a+ k=2∞vka sin kωt + θka 2.9

In the latter section, it will be shown how the series-APF can be designed to operate as a controlled voltage source whose output voltage would be automatically controlled according to the above equation.

The functions of the shunt active filter is to provide compensation of the load harmonic current, load reactive power demand and also to maintain dc link current constant. To provide load reactive power demand and compensation of the load harmonic and negative sequence currents, the shunt-APF acts as a controlled current source and its output components should include harmonic, reactive and negative-sequence components in order to compensate these quantities in the load current [6]. The per phase load current of shunt active filter is expressed as:

Ial=I1pmcos ωt – θ1 + Taln+k=2∞Ialk (2.10)

=I1pmcosωt cosθ1 + I1pmsin ωt sin θ1 k=2∞Ialk (2.11)

In order to compensate harmonic current and reactive power demand the shunt active filter should produce a current of:

Iah=I1pm+sin ωt sin θ1 +Ialn+k=2∞Iak (2.12)

Then the harmonic, reactive and negative-sequence current will not flow into power source. Hence, the current from the source terminal will be:

Ias=Ial-Iah=Ipmcos ωt – θ1 + Taln+k=2∞Ialk (2.13)

This is a perfect harmonic free sinusoidal current in phase with voltage.

2.4 Basic Control Function

It is evident from above discussion that UPQC should separate out the fundamental frequency positive sequence components first from the other components. Then it is required to control both series and shunt active filter to give output as shown in equations (2.9) and (2.18) respectively. The control strategy uses a PLL based unit vector template for extraction of reference signal from the distorted input supply. The block diagram of extraction of unit vector template is as given in Fig. 2.4.

vm

va,vb,vc vLa,vLb,vLc

The input source voltage at point of common coupling contains fundamental and distorted component. To get unit vector templates of voltage, the input voltage is sensed and multiplied by gain equal to 1/vm, where vm is peak amplitude of fundamental input voltage. These unit vector templates are then passed through a PLL for synchronization of signals. The unit vector templates for different phases are obtained as follows:

va=sin ωt

vb=sin (ωt-1200) (2.14)

vc=sin (ωt+1200)

2.5 Shunt Converter Control

The unit vector template of voltage is used to generate the reference signal for shunt APF. The control block diagram of shunt active filter is given in Fig. 2.5. As indicated earlier, the shunt APF compensates current harmonics in addition to maintaining the dc link current at a constant level. To achieve this, dc link current of the UPQC is compared with a constant reference current of magnitude equal to peak of harmonic current [10.]. The error between measured dc link current and reference current is processed in a PI controller.

Gatting

Signals

Ia Ib I

vavbvc Iar Ibr Icr

dc link

Pdc Ploss

Idc ref

The output of PI controller is added to real power loss component to derive reference source current given as:

vαvβ = 1/2 -1/2-1/203/2 -3/2 vavbvc (2.15)

IαIβ =1/2 -1/2-1/203/2 -3/2IaIbIc (2.16)

pt=vαtIαt+vβtIβt

qt=-vβtIαt+vαtIβt (2.17)

In matrix form it is given as:

pq = vαvβ-vβvα IαIβ (2.18)

From equation 2.18 the values of p and q can be expressed in terms of dc components plus the ac components as follows:

p=p+p

q=q+q (2.19)

Where

p is the dc component of the instantaneous power p, and is related to the fundamental active current.

p is the ac component of the imaginary power p, and is related to the harmonic current caused by the ac component of the instantaneous real power

q is the dc component of the imaginary instantaneous power q, and is related to the reactive power generated by the fundamental components of voltage and current

qis the ac component of the instantaneous imaginary power q, and is related to the harmonic current caused by the ac component of instantaneous reactive power.

To compute harmonic free unity power factor, three-phase currents, compensating powers pc and qc are selected as:

pc = pldc + ploss (2.20)

qc = 0

Where, plossis the instantaneous active power corresponding to the switching loss and resistive loss of UPQC. The total instantaneous active power is calculated by adding real power loss due to switching as shown in Fig.2.5. The orthogonal components of the fundamental current are obtained as follows:

IαIβ = vαvβ-vβvα pcqc (2.21)

The a-b-c components of fundamental reference current are obtained as follows:

i*sai*sbi*sc =2/30-1/31/3-1/31/3IαIβ (2.22)

The reference currents are then; compared with actual source current in a hystresis controller band to derive the switching signals to shunt inverter.

2.6 Series Converter Control

In order for the load voltage to be perfectly sinusoidal and balanced, the series filter should produce a voltage equal to equation (2.9). The reference load voltages are obtained by multiplying the unit vector templates with a constant equal to peak amplitude of fundamental input voltage. The compensation signals for series filter are thus obtained by comparing these reference load voltages with actual source voltage using equation (2.23).

v*fa=vsa-vmva v*fa=vsb-vmvb v*fa=vsc-vmvc (2.23)

The control of the series-active power filter is given in Fig. 2.6. The series-APF should behave as a controlled voltage source and its output should follow the pattern of voltage given in equation (2.9). This compensating voltage signal can be obtained by comparing the actual load terminal voltage with the desired value. These compensation signals are compared with actual signals at the terminals of series filter and the error is taken to hystresis controller to generate the required gating signal for series filter as shown in Fig. 2.6.

vla

v

v*fa Gatting

va signal

v*fb

vb v*fa

vlb vfa vfb vfc

Fig. 2.6 Control block diagram of series-APF

2.7 Modeling of UPQC

The three-phase system shown in Fig. 2.7 is considered for verifying the performance of UPQC. Three-phase source feeding this system at one end. For the best performance, UPQC is placed at the midpoint of the system as shown in Fig. 2.7. UPQC is placed between two sections B1and B2 of the transmission line. The complete system parameters are given in Table 2.1.

The STATCOM model in UPQC is connected in shunt with transmission line using step down transformer. the voltage can be regulated to improve the voltage stability of the power system. Thus the main function of the STATCOM is to regulate key bus voltage magnitude by dynamically absorbing or generating power to the ac transmission line.

The SSSC which is connected by series transformer with transmission line generates three-phase voltage of controllable magnitude and phase angle. This voltage injection in series with the transmission line is almost in quadrature with the line current and hence emulates an equivalent inductive or capacitive reactance in series with the transmission line. A small part of this injected voltage is in phase with the transmission line current supplying the required losses in the Inverter Bridge and transformer.

Three-phase AC source
Rated voltage	11 kV
Frequency	50 Hz
SC level	200 MVA
Base voltage	11 KV
X/R	8
Transmission line parameters
Resistance of the line	0.01273 Ω/km
Inductive reactance of the line	0.09337 mH/km
Capacitive reactance of the line	12.74 nF/km
DC link
Capacitance of DC link Capacitor	2500 μF
DC link voltage	700 V
Shunt transformer
Nominal power	50 MVA
Frequency	50 Hz
Primary voltage	11 kV
Secondary voltage	440 V
Magnetization resistance	50 p.u.
Magnetization reactance	50 p.u.
Series transformer
Nominal power	50 MVA
Frequency	50 Hz
Primary voltage	440 V
Secondary voltage	230 V
Magnetization resistance	20 p.u.
Magnetization reactance	20 p.u.

Table 2.1

Power system parameter

2.8 abc to dq Conversion in MATLAB

As shown in Fig. 2.8 (a), a discrete 3-Phase Programmable Source block is used to generate 1pu, 1200 degrees positive sequence voltage. At t = 0.1s an unbalance is introduced by adding a 0.3 p.u. negative sequence component with a phase shift of -300.

The Phase Locked Loop block measures the system frequency and provides the phase synchronous angle θ (more precisely [sin (θ), cos (θ)]) for the dq Transformations block. In steady state, sin (θ) is in phase with the fundamental (positive sequence) of the component.

Fig. 2.8 (b) shows the main conversion block used for conversion, the first block is three phase sequence analyzer; its function is to do Fourier analysis over one cycle of the specified frequency is first applied on the three input signals to find phasor va, vb and vcat fundamental or harmonic frequency. Then, transformation is applied to obtain the positive-sequence v1, negative-sequence v2 and zero-sequence v0. This block can be used in a control system to measure a positive sequence voltage or current.

The abc_to_dq0 Transformation block computes the direct axis, quadratic axis, and zero sequence quantities in a two-axis rotating reference frame for a three-phase sinusoidal signal. The following transformation is used:

vd= 2/3 (vasin (ωt)+ vb sin (ωt-2π/3)+vc sin (ωt+2π/3))

vq= 2/3 (vacos (ωt)+ vb cos (ωt-2π/3)+vc cos (ωt+2π/3)) (2.24)

vo=1/3(va+vb+vc)

Where ω = rotation speed (rad/s) of the rotating frame.

This block can be used to measure the positive-sequence component of a set of three-phase voltages or currents. The vd and Vq(or Id and Iq) then represent the rectangular coordinates of the positive-sequence component.

Math Function and Trigonometric Function blocks are used to evaluate the magnitude and phase of the positive sequence from the d and q components, using the Math Function block and the Trigonometric Function block the modulus and angle of voltage is obtained as :

Modulus:

| v| = [(vd)2 +(vq)2 ]1/2(2.25)

Angle:

∟v = atan2 (vq / vd)

The Math Function block shown performs numerous common mathematical functions, hypot indicates Square root of sum squares. The Trigonometric Function block performs common trigonometric functions. The name of the function appears on the block. Here, atan2 function is selected the block displays two inputs. In the Fig. 2.8 (b) first input is the y-axis or complex part of the function argument. The second input is the x-axis or real part of the function argument.

There is another block for dq to abc conversion. The dq0_to_abc Transformation block performs the reverse Park transformation, which is commonly used in three-phase electric machine models. It transforms three quantities (direct axis, quadratic axis, and zero-sequence components) expressed in a two-axis reference frame back to phase quantities. The following transformation is used:

va= ( vd sin (ωt)+vqcos(ωt)+vo )

vb= ( vd sin (ωt-2π/3)+vqcos(ωt-2π/3)+vo ) (2.26)

vc = ( Vd sin (ωt+2π/3)+vqcos(ωt+2π/3)+vo)

Where ω = rotation speed (rad/s) of the rotating frame.

2.9 Simulated Results

As shown in Fig.2.9 phase abc and synchronous reference frame quantity dq shown. As shown, unbalance created at 0.1s by adding 0.3 p.u. negative sequence component with phase shift of 300 hence significant changes in the magnitude of phase voltage and dq quantity occurs.

2.10 Shunt Controller/STATCOMin MATLAB

The controller shown in Fig. 2.10 is an integral part of the converter present in STATCOM to operate voltage control mode. Its function is to operate the rectifier power switches so as to maintain a fixed dc voltage in the dc link and to generate a fundamental output voltage waveform with demanded magnitude and phase angle in synchronism with the sinusoidal system which forces the reactive power exchange required for compensation.

The STATCOM controller has the capability of independently controlling the shunt real and reactive power components. In the automatic voltage control mode, the shunt converter reactive current is automatically regulated to maintain the transmission line voltage to a reference value at the point of connection. However, the shunt real power control is dictated by the dc voltage controller as shown in Fig. 2.11, which acts to maintain a preset voltage level on the dc link, thereby providing the real power supply or sink needed for the support of the series voltage injection. In other words, this dc voltage controller ensures the real power balance between the shunt and series converters.

In the scheme shown in Fig. 2.8 (a) and Fig. 2.8 (b) the three-phase voltage and current are sensed and transformed into two-phase quantities using Park's transformation, which gives d-q-axis current and voltage for the controller. The dc voltage controller calculates the reference value for the d-axis inner current controller.

As shown in Fig. 2.12, inner current controller is considered particularly suitable for current source rectifier due to its safety, stability performance and fast response. Typically the inner current control loop is at least ten times faster than the outer loop controlling the dc voltage. The Idrefobtained from the voltage controller is compared with the actual d-axis current and stabilized through PI controller to get the equivalent d-axis reference voltage vd. Similarly the actual q-axis current is compared with Iqrefand the error so obtained is stabilized through PI controller to get the equivalent q-axis reference voltage vq. The parameters of these PI controllers are tuned and fine adjustment is carried out by trial and error procedure to minimize the performance indices, namely the integral square error and integral time absolute error so as to give the best response.

The reference voltages vdand vqare compared with actual vdand vqto obtain the equivalent vdav, and vqav. Then these two-phase quantities are converted into three-phase quantities using dq-abc transformation. These three-phase voltages are fed as control signals to the PWM modulator for developing the switching pulses to the current source rectifier switches.

2.11 SeriesConverter/SSSC Modelin MATLAB

A SSSC is a solid-state voltage source inverter, which generates a controllable AC voltage source, and connected in series to power transmission lines in a power system. The injected voltage (vq) is in quadrature with the line current I, and emulates an inductive or a capacitive reactance so as to influence the power flow in the transmission lines. The compensation level can be controlled dynamically by changing the magnitude and polarity of vq and the device can be operated both in capacitive and inductive mode. The MATLAB modeling of control system of SSSC is shown in Fig. 2.13. The control system consists of:

1. A phase-locked loop (PLL) which synchronizes measured positive-sequence component of the current with self generated current. The output of the PLL (θ =ωt) is used to compute the direct-axis and quadrature-axis components of the AC three-phase voltages and currents.
2. Sequence of voltages v1 and v2 (V1q and v2q) as well as the dc voltage vdc.
3. AC and DC voltage regulators which compute the two components of the converter voltage (vdcnv and vqcnv) required obtaining the desired dc voltage (vdcref) and the injected voltage (vqref).

The variation of injected voltage is performed by means of a Voltage-sourced converter (VSC) connected on the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices (e.g. GTOs, IGBTs or IGCTs) to synthesize a voltage vcnv from a dc voltage source. A capacitor connected on the dc side of the VSC acts as a dc voltage source.

In the control system block diagram vdcnv and vqcnv designate the components of converter voltage vcnv which are respectively in phase and in quadrature with line current I. VSC using IGBT-based PWM inverters is used in the present study. Harmonics are cancelled by connecting filters at the AC side of the VSC.

This type of VSC uses a fixed dc voltage vdc. The converter voltage vcnv is varied by changing the modulation index of the PWM modulator.

2.12 Simulation Results of UPQCUsing PI Controller

An ideal three-phase sinusoidal supply voltage of 11kV, 50Hz is applied to the non-linear load (diode rectifier feeding an RL load) injecting current harmonics into the system. Fig. 2.14 (b) shows supply current in three phase before compensation from 0s to 0.1s, and after compensation from 0.1s to 0.4s. Shunt inverter is able to reduce the harmonics from entering into the system. The Total Harmonic Distortion (THD), which was 20.02% (Fig.2.20a) before compensation was effectively reduced to 4.04 % (Fig. 2.20b) after compensation using PI controller. It is clearly shown in Fig. 2.18 which shows single phase-phase b compensation. THD of all the waveforms discussed in detail in chapter 3.

The compensating shunt currents generated contain harmonic content of the load current Fig. 2.14 (a) but with opposite polarity such that when they are injected at the point of common coupling the harmonic content of supply current is effectively reduced. Reduced value is held constant using PI controller.

Fig. 2.14 (d) shows the source voltage with THD of 2.72%. Fig. 2.14 (c) and Fig. 2.14 (d) shows the load voltage and load currents respectively. The distortion due to non linear RL load. THD response of the line current and line voltage in the STATCOM side are found to be very low. Fig. 2.14 (c) shows load voltage.

Fig. 2.23 shows the bus voltage and current in the STATCOM side, which are found to be in phase with each other. This shows that the UPQC takes the role of phase angle compensation by absorbing or supplying the reactive power with the transmission line for any load variations. Fig. 2.26 shows dc capacitor voltage at 0.1s UPQC turned on and capacitor starts charging. Because of use of PI controller, capacitor charging takes some time and capacitor voltage also shows more oscillations in comparison to use of fuzzy logic controller. Here simulation is performed on 11kV line and the capacitor value used is 2500μF which is much lesser compare to actual requirement, hence dc capacitor shows more fluctuations.

When the transmission line is without UPQC, the real and reactive power flow cannot be controlled. Fig. 2.19 (a) shows the active power and Fig. 2.19 (b) shows reactive power through the line without UPQC from 0s to 0.1s after that with UPQC connected. The active power flow through line which is controlled by UPQC. Transmission capability of the existing transmission line is highly improved with the presence of UPQC. The difference between the sending-end real power and receiving end real power is high in the transmission line without UPQC. This is due to the increase in transmission losses, which are minimized with the help of UPQC as shown in Fig. Fig. 2.19 (c).It also helps in improving power factor of the transmission line. As shown in Fig. 2.19 (d), without UPQC, power factor of the transmission line is 0.93 but as UPQC switched, the power factor increases to 0.99. The reactive Power flow through the transmission line with and without UPQC is shown in Fig. 2.19 (b). The raise in the transmission capability is noticed from the simulation results.

The power transfer capability of long transmission lines is usually limited by their thermal capability. Utilizing the existing transmission line at its maximum thermal capability is possible with UPQC. The series inverter injects voltage of variable magnitude and phase into the transmission line at the point of its connection, there by controlling real and reactive power flow through the line. The active power through the line is supplied by SSSC active power. This real power obtained from the dc source connected to its dc terminals. The shunt inverter provides the required power to the series inverter through the dc link. For unbalanced condition extra RL load at dc side of the capacitor is connected at 0.4s. The response of active power, reactive power and terminal voltage is shown in Fig. 19 (a)- Fig.19 (d) .Unbalance is created by switching RL load on the ac side of the diode rectifier on phase a and at 0.5s and 0.6s another RL load at phase b and phase c connected respectively as shown in Fig. 2.7. It is obvious from the Fig.19 (a) - Fig. 19(d) that in unbalanced load condition, UPQC performs active, reactive compensation, phase angle regulation and harmonic filtering. Hence UPQC performance tasted under normal as well as unbalanced condition. Fig 2.21 (a) and Fig. 2.21 (b) shows the behavior of the PI controller when extra RL load is connected after 0.4s.With the use of PI controller, load current THD is reduced to 3.52% while THD of the source current is 1.89%.

Fig. 2.20 shows the voltage across dc link capacitor under various conditions. UPQC is switched at 0.1s, at the same instant dc capacitor starts charging and obtained certain voltage which cause active power transfer to series APF, after switching load at dc terminals of load diode rectifier, fluctuation in dc capacitor increases this is due to use of PI controller. When unbalance condition created by switching load to all three-phase one by one as explained earlier, dc capacitor voltage still maintained. This proves that UPQC is able to transfer active power through dc link capacitor in all the conditions.

Where Ps= Sending end active power, Pr =Receiving end active power, Ps= Sending end active power, Pr =Receiving end active power Vs =Sending end voltage, Vr =Receiving end voltag

2.13 Conclusion

This chapter presents control and performance of UPQC intended for installation on a transmission line with the help of PI controller. A control system is simulated in switching and unbalanced condition with shunt inverter and series inverter in open loop phase angle control mode. Simulation results show the effectiveness of UPQC in active filtering and controlling real and reactive power through the line.AC voltage regulation and power factor of the transmission line also improved. This chapter presents an improvement in the real and reactive power flow through the transmission line with UPQC using PI controller when compared to the system without UPQC.

Chapter 3: MODELING OF UPFC USING FUZZY LOGIC CONTROLLER

3.1 Introduction

Fuzzy Logic is a new control approach with a great potential for real-time application. Fuzzy logic controller is a rule based controller where a set of rules represents a control decision mechanism to correct the effect of certain causes coming from power systems [13].

In fuzzy logic, the linguistic variables are expressed by fuzzy sets defined on their respective universes. Error (input) can be selected as current, voltage or impedance, according to selected control type. The output of the fuzzy logic controller is the angle signal and the pulse generator provides firing pulses to thyristors.The fuzzy control is basically a nonlinear and adaptive in nature, giving the robust performance in the cases where in the effects of parameter variation of controller is present. It is claimed that the fuzzy logic controller yields the results which are superior to those obtained with the conventional controllers such as PI, PID etc. In the fuzzy controller, the simplicity of a PI controller is combined with the intelligent and adaptive ness of the fuzzy logic based control system [14]-[15].

Inputs to the fuzzy controller are categorized as various linguistic variables with their corresponding membership values as shown in the Table3.1. Depending upon the range (very large, large, medium, small and Zero) and the sign (positive or negative) of the error signals E1 and E2 , the FPI searches the corresponding output from the linguistic codes given in the Table 3.1. The simulation results using fuzzy controller are discussed in chapter 4.

3.2 Fuzzy Logic Implementation in UPQC Controller

As shown in Fig. 3.1 fuzzy logic controller block implemented instead of traditional PI controller in shunt controller Fig. 3.1, dc voltage regulating circuit Fig. 3.2 and series controller as shown in Fig.3.3.In order to achieve the desired response the PI controllers present in the control scheme are replaced by a fuzzy logic controller (FLC) whose membership functions are given in Fig. 3.4a, Fig 3.4b and Fig. 3.4c. The corresponding rule sets are given in Table 3.1.

In order to observe the performance of self regulated dc bus, the voltage across the capacitor is sensed at regular intervals and controlled by employing a suitable closed loop control. The dc link voltage, vdc is sensed at a regular interval and is compared with its reference counterpart vdc*. The error signal is processed in a fuzzy controller. A limit is put on the output of controller this ensures that the source supplies active power of the load and dc bus of the UPQC Fig. 3.2. Part of active power supplied by source is used to provide a self supported dc link of the UPQC. Thus, the dc bus voltage of the UPQC is maintained to have a proper current control

Table 3.1

Set of Fuzzy Rule Representation for FPI

E2 E1	PL	PM	PS	Z	NS	NM	NL
NL	PL	PL	PL	PM	PM	PS	Z
NM	PL	PL	PM	PM	PS	Z	ZS
NS	PL	PM	PS	Z	NS	NM	NL
Z	PL	PM	PS	Z	NS	NM	NL
PS	PM	PS	Z	NS	NM	NL	NL
PM	PS	Z	NS	NM	NM	NL	NL
PL	Z	NS	NM	NM	NL	NL	NL

As shown in Fig. 3.5, The surface viewer can generate a three-dimensional output surface where any two of the inputs vary, but two of the inputs must be held constant because computer monitors cannot display a five-dimensional shape. In such a case, the input is a two-dimensional vector with NaNs holding the place of the varying inputs while numerical values indicates those values that remain fixed. Because this curve represents a two-input one-output case, one can see the entire mapping in one plot [15].

3.3 Simulation Results and Discussion

Fig. 3.6a and Fig. 3.6b shows the source current and source voltage respectively. As shown in Fig. 3.6a, after switching of UPQC, source current becomes sinusoidal and from Fig. 3.7a the THD of compensated source current is 3.81% which is lesser compare to PI controller. The source voltage THD is 2.23% as shown in Fig. 3.7b. Fig. 3.6c is the dc link voltage (voltage across the dc capacitor) that feeds both the shunt and series inverters. The capacitor is effectively charged to the reference voltage, vdc drawing the charging current from the supply. Once it is charged to required value, it is held constant using fuzzy controller. There is no drop in the capacitor voltage when it feeds shunt inverter, because shunt inverter draws only reactive power to compensate the load current harmonics. When extra RL load is switched, the source current THD reduces to 3.26% as per Fig. 3.8a and source voltage THD is reduced to 1.26% as per Fig. 3.8b.

3.4 Conclusion

This chapter gives overview of fuzzy logic controller and its implementation in UPQC under switching and unbalanced conditionr. Triangular membership function with rule table is implemented using two input one output. Surface viewer is shown to evaluate the output response compare to two inputs. Fuzzy controller with use of the power flow as controlling input is designed in order to improve system's transient stability.

Chapter 4: COMPARISON OF PI CONTROLLER AND FUZZY CONTROLLER

4.1 Introduction

The simulation results of UPQC obtained using PI controller (obtained in chapter 2) and simulation results obtained using fuzzy logic controller (obtained in chapter 3)are compared in this chapter

4.2 Simulation Results and Discussions

Table 4.1 shows simulated performance parameters of PI controller and fuzzy logic controller. It is clearly evident from the Table 4.1 that fuzzy logic control having an edge over PI controller. Results shown in Table 4.1 are verified one by one.

Table 4.1

Simulation Results Obtained

	Factor	PI Controller	Fuzzy Controller
1	Source current THD	4.04%	3.81%
2	Dynamic response	Slow ( 0.20s)	Fast ( 0.10s)
4	Capacitor charging	Slower	Faster
5	Capacitor voltage balance under unbalanced load condition	Less stable	More stable
6	Source current THD with switching RL load	3.52%	3.26%

(1) Source current THD

As shown in Fig. 4.1, before compensation when UPQC not connected, source current THD is 20.02%, due to non linear RL load. The dominant harmonic is 5th harmonic and its magnitude is 18% of fundamental component. As shown in Fig. 2.15 in chapter 2, there is passive filter LC connected on shunt side which is tuned to 5th harmonic. Fig. 2.15b in chapter 2 shows source current THD after compensation when UPQC connected at 0.1s and PI controller used, source current THD is reduced to 4.04% and the magnitude of the 5th harmonic also reduces to 1% of fundamental component. But when PI controller replaced by the fuzzy logic controller, source current THD reduces to 3.81% as shown in Fig. 3.6a and Fig. 3.7a in chapter 3. And the magnitude of the 5th harmonic also reduces to 0.5% of fundamental component. So in the 1st, 3rd factor of Table 4.1, fuzzy controller proves to be more a advantageous.

(2) Dynamic response

This parameter is the measurement of how quickly controllers respond to the situation, in table 4.1 dynamic response (2) shows the time taken by the controller to reduce THD from 20.02% to 4.5%. as shown, time taken by PI controller is 0.20s and time taken by the fuzzy controller is 0.15s. Hence it is proved that dynamic response of th PI controller is faster than the fuzzy logic controller.

(3) DC capacitor voltage regulation

Fig. 2.20 in chapter 2 and Fig. 3.6c in chapter 3 is the dc link voltage that feeds both the shunt and series inverters. The capacitor is effectively charged to the reference voltage, vdc drawing the charging current from the supply. Once it is charged to required value, it is held constant using PI and fuzzy controller. There is no drop in the capacitor voltage. Fig. 2.20 shows the dc link voltage which reflects more the disturbance in the supply voltage because use of PI controller. But when fuzzy controller replaced, as shown in Fig. 3.6c, it shows less fluctuation and hence smoother exchange of real power between STATCOM and SSSC. From both fig. 2.20 and Fig. 3.6c, it can be seen that when UPQC switched at 0.1s, dc capacitor voltage using fuzzy controller quickly attains reference value compared to PI controller. In another condition, when extra RL load switched at 0.4s, fuzzy controller shows better response compare to the PI controller. This shows that capacitor voltage charging is faster in case of fuzzy controller. So the operating band of dc voltage limited to narrow range which is one of the salient nature of fuzzy logic controller. So in the 4th, 5th factor of Table 4.1, fuzzy controller proves to be more a advantageous.

(4) Source current THD with switching RL load

Fig. 2.21a and Fig. 2.21b in chapter 2 shows the source current THD after switching extra RL load in non linear diode rectifier. Fig. 2.21a shows Source current THD using PI controller and its value is 3.52%. While Fig. 3.7a in chapter 3 shows Source current THD using Fuzzy controller and its value is 3.26%. Fig. 2.21b and Fig. 3.7b shows the source voltage THD after switching extra RL load in non linear diode rectifier. Fig. 2.21b shows Source current THD using PI controller and its value is 1.89%. While Fig. 3.7b shows Source current THD using fuzzy controller and its value is 1.27%. So it is obvious that under switching condition, fuzzy controller gives better performance then PI controller. So in the 6th, 7th factor of Table 4.1, fuzzy controller proves to be more a advantageous under switching condition.

4.3 CONCLUSION

Simulated results of two control strategy of UPQC are discussed in detail with the help of comparison table. Comparison studies show that fuzzy logic controller is more advantageous in terms of compensation, dynamic response and capacitor voltage balancing. Simulated results are already discussed in detail.

Chapter 5: CONCLUSION AND FUTURE WORK

5.1 Conclusion

VAR compensation and harmonic filtering technique has gain tremendous interest over the years. Various topologies and control techniques have been reported in the literature. In this work various aspects of reactive power compensation and harmonic filtering is studied chronologically.

UPQC which combines the series and shunt active filter has been selected for further study in this work. Different control strategy of UPQC has been studied and one of the control strategies (dq method) has been studied in detail. The relevant simulations studies of UPQC have been carried out using MATLAB under various conditions. One of the disadvantages of the dq theory is that it requires a PI controller minimizing the error between the sensed quantity and reference quantity. However, the tuning of the PI controller is cumbersome and time consuming job.

In the present work fuzzy logic controller has been proposed in place of conventional PI controller. Fuzzy logic controller is non linear and adaptive in nature which gives the best performance under varying condition. Further no frequent tuning required so it is less time consuming and it is more accurate method then PI controller.

Results obtained from the simulation shows better performance of UPQC when fuzzy logic controller used then that of PI controller in terms of harmonic compensation and dc capacitor voltage balancing at load terminals in switching as well as unbalanced conditions. Under this conditions the dynamic response of fuzzy logic controller proved to be faster than PI controller. Hence it is proved that fuzzy logic controller is superior then PI controller.

UPQC and its method of control are the new area of concern. Present work focuses on two different control strategies PI control and fuzzy logic control. This work proves the advantages of the fuzzy logic control over traditional PI controller; fuzzy logic control of UPQC is the most recent area of concern. This work contributes towards fuzzy implementation in UPQC and the results discuss improvement of UPQC characteristics using fuzzy logic control.

So performance of UPQC is evaluated using MATLAB/Simulink with 11kV transmission line. In this scheme the fuzzy controller provides better results than traditional PI controller in both switching and varying load condition.

5.2 Scope Future Work

1. New fuzzy PI control strategy can be implemented using both fuzzy and PI controller simultaneously
2. To test the system with different membership function apart from triangular.
3. To develop new membership function and rule base table and implement in UPQC